Field of the Invention
The present invention relates to providing illumination for image storage and manipulation systems and, more particularly, to such systems concerned with images of objects moving through scenes being sensed by such systems.
There are many uses for systems which capture electromagnetic radiation images of scenes, including light images, and then perform manipulations on the images or their representations after such capture to detect or identify one or more aspects of such scenes. Such uses include surveillance for security purposes, sensing for robots or other controlled machines, inspection of results from operations performed on objects, sorting various kinds of objects for quality, size, and the like, identification, etc.
Among several of these uses, the situation arises where the scenes from which images are to be obtained will have objects of interest moving therethrough at a relatively fast rate with respect to the system acquiring the images of that scene. In many of those instances, instances which often arise in the operation of sorting or inspection, there is no particular interest in following the object through its path of travel, but rather a single sharp image is needed of each object passing through the scene, i.e., that which is on the sensing range of the system. Having acquired such an image, the system can then perform various pattern recognition manipulations to ascertain aspects of the object that are of interest.
If the object moves rapidly enough through the scene being sensed by the system, the image obtained will be blurred rather than sharp because the object will move appreciably during the time that the scene image is being acquired. The transducer typically used in acquiring an image of the scene is a television camera which provides video data in the RS-170 format, a standard promulgated by The Electronics Industry Association (EIA). Under this standard, a scene is represented as a series of images, or frames, one such image being acquired every thirtieth of a second. Each such image is acquired by having the scene illuminated such that the resulting reflected illumination from the surfaces in the scene is arranged to reach the image reception plane of the camera where the resulting image is converted into electrical signals. This conversion takes place at the image reception plane on the basis of considering that plane divided into five hundred twenty-five horizontal lines such an electrical signal is provided which is a sequence of analogs of the light intensity occurring on each of these lines due to the image occurring there of the scene.
The RS-170 standard requires that the signals representing images obtained by the camera be arranged so that they can control a television receiver, or monitor, operated by having an electron beam therein periodically scan five hundred twenty-five lines on a fluorescent material image reconstruction surface. Electrical signals representing the image are arranged to have every other line scanned first to form one field, and thereafter the remaining lines scanned to provide the second field, the electrical signals from these two fields being interlaced to recreate the scene in the frame based providing each of these two fields on the image reconstruction surface. This scanning, to provide two video fields for a video frame, is repeated thirty times a second (fields are thus scanned sixty times a second) to provide what appears to be a continuous image on television monitor, but what appears as a series of frames to an image storage and manipulation system.
The electrical signals from the camera representing the image are in the form of a sequence of signal portions each of which is the analog of, and represents, the light intensity occurring on a line of the image reception plane. Members of this sequence are separated by horizontal blanking and synchronism pulses which indicate separations between electrical signals resulting from each horizontal line scan, and vertical blanking and synchronism pulses which indicate the separation between the series of signals representing each scanning field, or each vertical scan, again with every two successive such video fields forming a video frame.
With each video frame taking approximately a thirieth of a second to assemble in terms of forming an electrical signal representation of every other line of the image applied during that time, objects which can move appreciably in the scene in such time durations will appear blurred in such an arrangement. A method for overcoming this blurring would be to acquire an image from the moving object in a short burst of illumination so that the camera could effectively sense the scene only during that period of time the illumination burst occurs. If the camera is set to take in relatively little illumination and the burst of illumination is quite bright relative to the light taken in in the absence of such a burst, the camera, in effect, will take a picture of the scene only during the burst.
If the duration of the burst of light is quite short so that a moving object does not move appreciably during that duration, the object will appear to not move past the camera during the frame so that with a sufficiently bright burst of light a sharp image will be obtained of the entire scene without any blurring due to the moving object. This requires a camera, however, that can store the image acquired during the burst for the rest of the frame. Such a camera could be a charge-coupled device based image reception plane camera.
A further problem which must be solved is that the burst of illumination must be made to occur at the time the moving object is in the desired position in the scene for purposes of having its image acquired. A possible way of controlling a burst of illumination is to have the source thereof initiate an illumination burst once in each frame based on a signal from the camera, or the image storage manipulation system, which is provided at a fixed point during the scanning period in which the frame is obtained. Such a stroboscopic illumination arrangement means, however, that the light is provided in synchronism with the frame rate of the camera rather than being based on the presence of the moving object at the desired image point in the scene a the moment an illumination burst is initiated.
As a result of such frame rate synchronism, the object would be at an unknown point with respect to the scene at the time the scene image is acquired. For a sequence of objects moving through the scene, such as objects to be inspected on a conveyor belt, each successive object could appear at a different point in the scene because its arrival on the scene and the initiation of a burst of illumination are not synchronized with each other. Such a result can lead to problems in providing a complete or properly focused image for acquisition by the image storage and manipulation system, which can lead to poor results in sorting or inspecting such objects. A desirable arrangement would be to synchronize the burst of illumination with the arrival of the moving object at a desired point in the scene.
A further difficulty arises even for a system which is able to provide such a desirable synchronization. Television cameras which acquire an image of the scene, and provide electrical signals representing it, have points during a scanning period for forming a frame in which they are unable to provide electrical signals representing the image. Should a burst of illumination occur during such a period of time, the image for that frame could be distorted or lost. In a camera relying on a charge-coupled device image reception plane as the place where an image is acquired and transformed into representative electrical signals, the point in a scanning period where the accumulated charge (accumulated by the photoreceptors because of an image portion following thereon) is placed into the transfer registers for shifting out to form the corresponding image signals, or video data, is a point in which an image falling on the photoreceptors cannot be reliably transformed into electrical signals. Thus, a system providing the desired illumination-moving object synchronization must also avoid having the illumination burst occur during such a point in time at which the camera cannot provide electrical signals representative of the image caused by the illumination burst.